Mechanic can't find the fault - how much is reasonable?

[Condi]

Long story short a few weeks ago my bike went into the local mechanic because it was running poorly when warm. They cleaned out the throttle bodies, replaced the spark plugs and told me it was fixed, with a £300 bill. The ride home was only short but the next day I took it out for a longer ride and it quickly became obvious that the fault was still there when it warmed up and just as bad as it was before. Took it straight back and they accepted that it was "almost unrideable" (their words). That was nearly 3 weeks ago. When I popped my head in the other day they admitted that they had no idea what was wrong, but had tried various things to no avail. At this point I don't have much confidence in them finding the problem, but would really like the bike fixed and back in the garage.

I'm starting to get concerned they have had the bike for over 2 weeks, and there is a big labour bill coming my way for little progress in identifying or fixing the problem. What would be the best course of action going forwards? Do I pick the bike up, pay the bill and take it elsewhere? Do I leave it with them in the hope they find the problem? Do I have an argument with them that having already paid £300 I'm refusing to pay for the additional work as there is no progress towards a solution? On the one hand they have put in some hours towards fixing it, but on the other I shouldn't be paying for them to sit there and scratch their chin while drinking tea.

[RazerSauber]

My first port of call would be speaking to them. Ask them if they'd prefer you removing the vehicle if they can't find out what's wrong with it. They might be glad to see the back of it. I wouldn't mention payment either, and wait to be asked for it.

[carinaman]

What bike is it?

Is it a modern one with a computer?

[Condi]

2001 Fireblade, so there is a computer, but a very basic first generation ECU.

[trickywoo]

It shouldn’t be that hard to find. Could very well be a coil pack.

After £300 and no fix you definitely need a chat with them before they have it any longer.

If it was me I’d do my best to get the bike back without paying any more money and take it somewhere else.

[Drabbesttunic]

Fuel pressure regulator can kick up problems like described.
Might be worth a chat to see what they've checked or looked for too and go from there.

[Vantagemech..]

Have they said what they have checked so far or is it a wet finger in the air diagnosis?

Had this with a vehicle in for MOT, sudden missfire doe they changed an injector and a few other bits before admitting it had dropped a valve while being tested and a replacement engine was the only remedy..... cost spiralled to more than the vehicles worth.

You need compression and leak down figures - a tight valve could give you these symptoms but is easily overlooked / too involved for some places.

[Condi]

I will see what they've checked, I know the coils themselves have been checked, swapped around etc and that wasn't the issue. They seem to be rounding on the idea its an ECU fault.